The present invention relates to a burner capable of suppressing or reducing production of nitrogen oxide (referred to as NOx hereinafter) in combustion gas, and more particularly, it relates to a low NOx burner for pulverized coal capable of remarkably reducing the NOx production during combustion of the pulverized coal.
Fossilization coal includes nitrogen (N) as well as combustible components such as carbon, hydrogen and the like. Particularly, coal includes a relatively large amount of the nitrogen, unlike gaseous fuel or liquid fuel. Therefore, an amount of the NOx production generated during combustion of the coal is more than that generated during combustion of the gaseous fuel. Thus, it has been desired that such NOx production is reduced or suppressed to the utmost.
NOx generated during combustion of various fuel is grouped into thermal NOx and fuel NOx, on the basis of the cause of its generation. The thermal NOx is generated by oxidization of nitrogen in the firing air; on the other hand, the fuel NOx is generated by oxidization of nitrogen in the fuel.
In order to suppress production of such NOx, various combustion methods have been proposed; one of the representative conventional combustion methods is a staged combustion method in which the firing air is supplied in lots for every stage. Another representative conventional combustion method is an exhaust gas recirculating method in which the exhaust gas having low concentration of oxygen is supplied into a combustion area. A common principle regarding these conventional low NOx combustion methods resides in the matter that reaction between nitrogen and oxygen is suppressed by lowering a flame temperature. However, NOx which can be suppressed by lowering the flame temperature is the thermal NOx; the generation of the fuel NOx is scarcely influenced by the flame temperature. Therefore, the combustion method in which the NOx production is suppressed by lowering the flame temperature is effective merely for the combustion of the fuel containing a low percentage of nitrogen. However, this conventional combustion method is not effective in the combustion of coal, since the NOx generated by the combustion of coal contains about 80% of the fuel NOx, as clarified by D. W. Pershing and J. O. L. Wendt (refer to "The influence of flame temperature and fuel NOx; The Sixteenth Symposium (Inter-rational) on Combustion, P389-399. The Combustion Institute, 1976").
Combustibles in the coal can be grouped into volatile matter (component) and solid component. According to this inherent nature of the coal, the combustion method for the pulverized coal includes a process for pyrolyzing the pulverized coal in which the volatile matter is volatilized or discharged, and a combustion process for burning the combustible solid component (referred to as char hereinafter) after said pyrolysis. Combustion rate of the volatile matter is higher than that of the solid component, and thus, the volatile matter is burned up in an early stage of the combustion. In the above pyrolysis process, nitrogen (N-component) contained in the coal is separated into N-component devolatilized together with the other volatile matter, and N-component retained in the char. Thus, the fuel NOx generated during combustion of the pulverized coal includes NOx obtained from the volatile N-component and NOx obtained from the N-component retained in the char.
However, as pointed out by D. W. Pershing and J. O. L. Wendt, in the case of the combustion of coal, the greater part of the NOx production is NOx obtained from the volatile matter (i.e., the fuel NOx). In view of this fact, it is required to solve a problem regarding the fuel NOx in the combustion of the coal.
It is known that the volatile N-component forms compounds such as NH.sub.3, HCN and the like in an early stage of the combustion and in a region wherein oxygen is insufficient. These nitrogenous compounds not only produce NOx by reacting with oxygen but also act as a reducing or deoxidizing agent for resolving NOx into nitrogen by reacting with the produced NOx. This reducing reaction of NOx with the nitrogenous compound proceeds when such compound co-exists with NOx; if the nitrogenous compound does not co-exist with NOx, the greater part of the nitrogenous compound is oxidized to produce NOx. Further, under the high temperature circumstances such as the combustion, this reducing reaction is liable to proceed, as a percentage of oxygen contained in the surrounding atmosphere decreases. Accordingly, in order to suppress the generation of NOx during combustion of coal, it is a technical key how to create such atmosphere containing a low concentration of oxygen (i.e., low oxygen region).
As described in the Japanese Utility Model Laid-Open No. 94004/1982, the Japanese Patent Publication No. 30161/1980 or the literature (D. M. Zallen, R. Gershman, M. P. Heap and W. H. Nurick, "The Generalization of Low Emission Coal Burner Technology" Proceedings of the Third Stationary Source Combustion System, volume II, p. 73-109, 1976), a conventional burner for forming a low oxygen region in a flame, known to date, is a burner for delaying the mixing of excessive air with a fuel rich flame by arranging a secondary firing air nozzle or a tertiary air nozzle remotely from a fuel nozzle.
In the above conventional burner, a secondary or tertiary air is injected as a straight advance jet from the air nozzle radially spaced apart from an outlet of the fuel nozzle. Therefore, in the combustion by means of this conventional burner, it is easy to form the low oxygen region in the fuel rich flame, since the mixing of the excessive air with the fuel rich flame is delayed; however, as the mixing is delayed, combustion time is lengthened, thus worsening combustion efficiency. Further, the above conventional burner has another disadvantage of having a large-sized combustion installation.